Oil film imaging apparatus

ABSTRACT

An apparatus for forming an image, comprising a photoconductive insulating layer having a thin liquid film thereon and separated from an electrode by a small gap. After an electrical field has been established between the photoconductive layer and the electrode, the photoconductive layer is selectively illuminated causing liquid transfer in imagewise configuration onto the electrode. An image is also formed on the photoconductive insulating layer. Alternately, a thin liquid film is applied to both the photoconductive insulating layer and the electrode or solely to the electrode.

United States Patent Inventor William L. Golle Webster. N.Y.

Appl. No. 8.134

Filed Jan. 16, 1970 Division of Ser. No. 571.343. Aug. 9. 1966 PatentedNov. 9, 1971 Assignee Xerox Corporation Rochester, N.Y.

OIL FILM IMAGING APPARATUS 6 Claims, 2 Drawing Figs.

US. Cl 355/16. 96/1 Int. Cl ..G03g15/14 Field ofSearch 35513.16.

Relerences Cited UNITED STATES PATENTS 2,904,431 9/1959 Yeates .1 355/17X 2,975,052 3/1961 Fotland et a1 96/1 3,254,998 6/1966 Schwertz 355/11 X3,317,316 5/1967 Bean et al.... 355/3X 3.383.993 5/1968 Yeh .1 355/3Primary Examiner-Samuel S. Matthews Assislan! Examiner- Fred L. BraunAttorneys-James .l. Ralabate. Norman E. Schroder and Ronald Zibelli OILFILM IMAGING APPARATUS This application is a divisional of application,Ser. No. 571,343 filed Aug.9, 1966.

This application relates to a novel means and method for imagereproduction in which neither chemical reactions nor particulatematerials are necessarily involved.

The invention will be described with relation to the drawing in whichFIG. 1 is a schematic cross-sectional view of apparatus used in carryingout the invention, and FIG. 2 illustrates a method of precharging.

Two platelike electrodes 10 and 12 are employed, each having at leastone flat surface. The fiat surface of electrode 12 is covered with athin layer, illustratively on the order of 0.002 inch thickness, of aphotoconductive-insulating material 14. Such a material is one which issufficiently resistive in darkness to be classified as a true electricalinsulator but which becomes relatively more conductive upon exposure toradiation such as light. A vacuum-evaporated layer of vitreous seleniumis the most common material of this classification and is a preferredmaterial for use in the invention, but other materials with similarproperties are known to the art and may be employed, and at lease one ofthese, a dispersion of zinc oxide in a resin binder, enjoys substantialcommercial use. in one embodiment of the invention, electrode 10 iscovered with a thin layer of insulating material on the order of a fewmicrons in thickness. Either photoconductive insulating layer 14 or thefacing surface of electrode 10 or both of them, in

that order of preference, is covered with a thin continuous layer of oil16. The term oil as employed in this specification relates to any liquidmaterial having a sufficiently low vapor pressure to form stable thinfilms and having an electrical resistivity sufficient to maintainpotential gradients along the surface of the film. The films employed inthe present invention are so thin that the resistivity required is metby most liquids but the vapor pressure requirement becomes morecritical. Ester-type fluids sold for use as vacuum diffusion pump oilsunder such names as Octoil" (di-octal phthalate) made by BendixCorporation Vacuum Division and Narcoil" (di-nonyl phthalate) made byNational Research Corporation are particularly suitable for use in theinvention. Oil film 16 must be very thin, preferably less than about 1or 2 microns. Films of this thickness range can be recognized by theinterference fringes which they produce.

A simple way to form the required oil film 16 is to separate electrodes10 and 12 and rub a dilute mixture of the selected oil in a volatilesolvent, such as acetone, onto the surfaces with a piece of cotton wool.The resulting liquid film will be much thicker than desired but thesolvent will rapidly evaporate and reduce the film thickness to thedesired range.

After the oil film 16 has been formed, electrodes 10 and 12 arepositioned as illustrated. The gap 18 between electrode 10 andphotoconductive insulating layer 14 should be on the order of 2 to 7microns and preferably not more than about 0.001 inch. This spacing canbe achieved in a practical manner by using thin spacers, not shown,between the electrodes l and 12 or between the electrode and the outeredge of photoconductive insulating layer l4. Strips of 0.00025 "Mylar"polyester film are effective spacers for use in the latter position. ADC power supply 20 and switch 21 is connected between electrodes 10 and12 and supplies a voltage in the range of 500-l,200 volts and generallyabout 600 volts in order to create an electric field in gap 18.

The only further step required to form an image is to selectivelyilluminate photoconductive-insulating layer 14 with an image pattern oflight. To accomplish this, either electrode 10 or electrode 12 should beoptically transparent. For this purpose, the electrodes may comprisesheets of transparent material with a thin transparent surface coatingof an electrically conductive material. Glass coated with a thintransparent conductive layer of tin oxide is a preferred material and isavailable commercially under various trade names. The elec trodes mayalso be constructed by applying very thin metallic films or films ofcopper iodide to sheets of transparent material such as glass orplastics.

l have found that with the apparatus of FIG. 1 less exposure is requiredto satisfactorily illuminate photoconductive insulating layer 14 throughelectrode 12, and accordingly there is shown an original subject 22illuminated by lamps 24 and which is focused by lens 26 throughtransparent electrode 12 onto the back of photoconductive insulatinglayer 14. Contact exposure may also be employed. However, I have alsosuccessfully carried out my invention by exposing layer 14 through atransparent electrode 10. When layer 14 is selectively illuminated, avisible image appears within a few seconds at most on each ofphotoconductive insulating layer 14 and electrode 10. The mechanism ofimage formation is not fully understood, but it appears that microscopicdroplets of oil breakoff from the oil films and are transported acrossthe gap 18 under the control of the pattern of resistivity in layer 14as caused by the selective illumination from subject 22. if the oilfilms are thicker than indicated, large globules will transfer and fonnan image pattern of varying film thickness. lf gap 18 is thicker thanindicated, there will be a loss in resolution. For similar reasonsinsulating layer ll, if used, should have a thickness in the order ofmicrons.

lf layer 14 is illuminated through electrode 12, as shown, and electrode10 is also transparent, then a visible image is immediately viewable byweak illumination through electrode 10 because photoconductiveinsulating layer 14 appears less sensitive to illumination throughelectrode 10 than through electrode l2. lf, however, a bright light isshown through electrode 12 or if subject 22 is replaced by a sheet ofwhite paper, for example, the image will be erased and a new image canbe formed later. When the original exposure conditions are restored theimage will reappear. if the illumination is stopped or switch 21 isdisconnected or electrodes 10 and 12 are separated, a reasonable stableimage will remain because extremely thin oil films such as I employ arenot readily selfleveling. Once power supply 20 is disconnected orelectrodes 10 and 12 are separated, ambient illumination will have noeffect on the images. The images are readily visible as an interferencepattern and may be projected. Since the images are in the form of a thinliquid film they may be transferred by rolling to a sheet of paper orthe like. The transferred image is not itself visible unless it has beentreated with a first chemical which reacts with a second chemical in thepaper to form a color reaction. Color reaction imaging per se is wellknown. The invisible transferred image can also be made visible bydusting the sheet with a finely divided powder which will selectivelyadhere to the areas having the greatest amount of oil. Such adevelopment technique may also be employed to enhance the visibility ofthe oil image directly on electrode 10 or layer 14, but the dusted layeror electrode cannot be reused until the powder is washed off.

FIG. 2 illustrates a method of precharging photoconductive insulatinglayer 14. Layer 14 is passed beneath a coronagenerating device 30 whichis maintained at a potential of several thousand volts relative toelectrode 12 by a high-voltage DC power supply 32. if this operation iscarried out in darkness, photoconductive insulating layer 14 can becharged to a high-surface potential, illustratively about 600 voltspositive for selenium layers. The charging operation can be carried outafter an oil film 16 has been formed on layer 14. After charging, theapparatus may be assembled in the manner shown in FIG. 1, except thatpower supply 20 can be of much lower voltage or even be replaced by asimple connecting wire. since the surface potential on layer 14 createsthe necessary electric field in gap 18.

Using the apparatus of FIG. 1, the application of voltage from powersupply 20 for a time as shown as one-half second or less is sufficientto form an image. If no insulating layer 1 1 is present, it appears thattiny droplets of oil are carried back and forth across gap 18 inilluminated areas, causing a redistribution of oil film 16 as originallylaid down. It is postulated that there is an air breakdown occurringwithin the gap and that ion deposition on the oil film may locallydestroy the surface tension and permit droplet formation in the presenceof the electric field, which is strongest in illuminated areas. When theexposure intensity is increased to very high levels a reversal of imagepolarity is observed.

When an insulating layer 11, such as Staybelite" rosin ester, isemployed on electrode 10, the apparatus is more sensitive to light andoil transport across the gap takes place in nonilluminated areas,instead of an illuminated areas when insulating film 11 is absent. It isbelieved that charge transfer takes place to the insulating layer inareas of exposure and that the potential built up on insulating layer 11 reduces the field in the gap and prevents transport of oil across thegap. 1f the apparatus of FIG. 1 is tested without oil film 16 but withinsulating film 11 an electrostatic charge pattern is observable oninsulator ll after the electrodes have been separated. By using theapparatus of FIG. 1 in connection with an insulating film 11, comprisinga few microns of Staybelite," image resolutions of 47 lines as permillimeter have been achieved with storage life times in excess of aminute. For maximum resolution, all film and gap thicknesses should bekept as small as possible. Under optimum conditions the amount of oiltransferred across the gap is very small, but is sufficient to form avisible interference pattern or for use in the other previouslydescribed ways.

While the present invention has been particularly described in terms ofa specific embodiment thereof, it will be understood that in view of theforegoing specification numerous deviations therefrom and modificationsthereupon may be readily devised by those skilled in the art withoutdeparting from the present teaching. Accordingly, the present inventionis to be broadly construed and limited only by the scope and spirit ofthe claims now appended hereto.

What is claimed is:

l. Image-forming apparatus comprising:

a photoconductive-insulating layer supported on a first electrode;

a second electrode positioned substantially parallel to saidphotoconductive-insulating layer and defining a gap of less than 0.00linch therefrom;

a nongap-bridging nonvolatile-insulating liquid interference filmpositioned on at least one of said photoconductiveinsulating layer andsaid second electrode;

DC power supply means connected between said first and secondelectrodes; and

means to project an optical image on said photoconductiveinsulatinglayer, whereby an image pattern of said liquid is formed on said secondelectrode and said photoconductive-insulating layer.

2. The apparatus of claim 1 in which said first electrode is transparentand said optical image is projected through said first electrode ontosaid photoconductive-insulating layer.

3. The apparatus of claim 2 in which said second electrode is coveredwith a thin layer of insulating material.

4. The apparatus of claim 1 in which said second electrode is coveredwith a thin layer of insulating material.

5. The apparatus of claim 1 in which said second electrode istransparent and said optical image is projected through said secondelectrode and said gap onto said photoconductive insulating layer.

6. The apparatus of claim 5 in which said second electrode is coveredwith a thin layer of insulating material.

i I i t

2. The apparatus of claim 1 in which said first electrode is transparentand said optical image is projected through said first electrode ontosaid photoconductive-insulating layer.
 3. The apparatus of claim 2 inwhich said second electrode is covered with a thin layer of insulatingmaterial.
 4. The apparatus of claim 1 in which said second electrode iscovered with a thin layer of insulating material.
 5. The apparatus ofclaim 1 in which said second electrode is transparent and said opticalimage is projected through said second electrode and said gap onto saidphotoconductive insulating layer.
 6. The apparatus of claim 5 in whichsaid second electrode is covered with a thin layer of insulatingmaterial.